Zero field splitting describes various interactions of the energy levels of an electron spin (S>1/2) even in the absence of an applied magnetic field. It is important in the electron spin resonance of biological molecules.
The value of the ZFS parameter is usually defined as D. Values of D have been obtained for a wide number of organic biradicals by EPR/ESR measurements. This value may be measured by other magnetometry techniques such as SQUID, however EPR measurements provide more accurate data in most cases.
For an S=1 spin system, in the presence of a magnetic field, the levels with different values of magnetic spin quantum number (MS=0,±1) are separated and the Zeeman splitting dictates their separation. In the absence of magnetic field, the 3 levels of the triplet are isoenergetic to the first order. However, when we incorporate the effect one electron has on the other, the energy of the sublevels of the triplet are separated out (zero field splitting). The separation of these sub levels manifest in peaks of the frozen solution EPR spectrum.
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